Shaped articles from poly (p-methylstyrene) blends

ABSTRACT

Shaped articles of a blend of PPMS and sensitizer such as triallyl cyanurate or isocyanurate are provided. They are thermoformed and irradiated with sufficient ionizing radiation to effect sufficient cross-linking to render the shaped article resistant to fatty foods under microwave oven conditions. Claims are also directed to the method.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to U.S. application Ser. No. 183,600, filedSept. 2, 1980 in the name of Arthur B. Robertson, now U.S. Pat. No.4,351,884, issued Sept. 28, 1982, which relates to shaped articles of ablend of PPMS and triallyl cyanurate or isocyanurate. They arethermoformed and irradiated with sufficient ionizing radiation to effectsufficient cross-linking to render the shaped article resistant to fattyfoods under microwave oven conditions. Claims are also directed to themethod.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to shaped article of poly(p-methylstyrene)blends and with such blends which can be cross-linked by ionizingradiation at low dosage levels.

2. Description of the Prior Art

In U.S. Pat. No. 4,205,114 there are disclosed shaped articles ofpoly(p-methylstyrene) that are cross-linked by ionizing radiation torender such articles resistant to fatty foods under microwave ovenconditions. The amount of radiation required to cross-linkpoly(p-methylstyrene), however, is between about 50 megarads and about60 megarads. This is too high to be commercially feasible. Thedesiteratum is to be able to effect the cross-linking at lower, morefeasible dosage levels.

SUMMARY OF THE INVENTION

This invention provides a shaped article of a blend ofpoly(p-methylstyrene) and sensitizer thermoformed and irradiated withsufficient ionizing radiation to effect cross-linking sufficient torender the shaped article resistant to fatty foods under microwave ovenconditions.

It also provides a method of making shaped articles that are resistantto fatty foods under microwave oven conditions, that comprises forming ablend of poly(p-methylstyrene) with an effective cross-linking amount ofa sensitizer into a foamed or unfoamed sheet, thermoforming said sheetto produce a shaped article, and irradiating the shaped article withsufficient ionizing radiation to effect cross-linking sufficient torender the shaped article resistant to fatty foods under microwave ovenconditions.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The monomer used in preparing the homopolymer or copolymers from whichthe blends of this invention are made is p-methylstyrene. It is withinthe contemplation of this invention to use mixtures of methylstyrenerich in p-methylstyrene. Such mixtures contain at least about 90 weightpercent, preferably at least about 95 weight percent, p-methylstyreneand less than 0.1 weight percent o-methylstyrene with the balance beingm-methylstyrene. A typical mixture contains, by weight, about 95 percentp-methylstyrene, about 5 percent m-methylstyrene, and about 0.05 percento-methylstyrene. The mixtures are obtained by catalytic dehydrogenationof the mixtures of ethylmethyl benzene isomers described in U.S. Pat.No. 4,086,287 to which reference is made.

The polymers contemplated herein are p-methylstyrene orp-methylstyrene-rich isomer mixture homopolymer or their copolymerscontaining between about 10 and about 1 weight percent conjugated diene,such as butadiene and isoprene. The polymerization reaction is carriedout by using methods and catalysts well known in the art forpolymerizing styrene. The reaction can be carried out in solution, bulk,suspension, or emulsion.

In accordance with this invention, the material that is blended with thepoly(p-methylstyrene) is a sensitizer. The sensitizer which is blendedwith the impact poly(p-methylstyrene) may be one or more multifunctionalvinyl monomers. Examples of such multifunctional vinyl monomers includetriallyl cyanurate, triallyl isocyanurate, 1,6-hexanediol diacrylate,tetraethylene glycol diacrylate, trimethylolpropane triacrylate,pentaerythritol triacrylate, 1,6-hexanediol dimethacrylate,tetraethylene glycol dimethacrylate, trimethylolpropane trimethacrylate,pentaerythritol trimethacrylate, and polyunsaturated fatty acids such aslinseed oil. Small amounts of sensitizer are effective to inducesufficient cross-linking by radiation at low dosage level. Generally,between about 0.1 weight percent and about 10 weight percent, preferablybetween about 0.5 weight percent and about 5 weight percent, will beused. Blending can be effected in a variety of ways, such as in solutionin aromatic solvents, such as toluene, from which sheets or films can becast. The blend components can be mixed in a tumbling agitator and fedinto an extruder to produce sheets, films, or foamed sheets, or they canbe fed separately to an extruder for extruder blending.

The blends can be formed into sheets, films, or foamed sheets andthermoformed to produce shaped articles, such as food containers. Afterbeing irradiated, such shaped articles are heat resistant and are notaffected by fatty foods under microwave oven conditions.

Within the contemplation of this invention, "shaped articles ofpoly(p-methylstyrene) blend" refers to the blend ofpoly(p-methylstyrene) with sensitizer. This includes shaped articlesmade from foamed poly(p-methylstyrene) blend sheet, from biaxiallyoriented poly(p-methylstyrene) blend sheet, from biaxially orientedpoly(p-methylstyrene) blend film laminated with biaxially orientedfoamed poly(p-methylstyrene) blend sheet, or from biaxially orientedpoly(p-methylstyrene) blend film laminated with foamed polystyrenesheet.

The manner by which foamed sheet is prepared is not an essential featureof this invention. The general methods of forming foamed sheet arediscussed in U.S. Pat. No. 3,444,283 which is incorporated herein byreference. The preferred method is a direct injection extrusionoperation described in U.S. Pat. No. 3,444,283. Reference is made hereinto U.S. Pat. No. 3,619,445, incorporated herein by reference, whichdescribes the complete direct injection extrusion foam lines. Ser. No.506,836, referred to in U.S. Pat. No. 3,619,445, is now U.S. Pat. No.3,444,283. As described in these incorporated patents, nucleating orcell size control agents can be used.

The invention is also applicable to injection molded articles made frompoly(p-methylstyrene) blend resin.

The poly(p-methylstyrene) blend sheet contemplated herein can beproduced by any known technique for making oriented polystyrene sheet orfilm. A feasible method is described in U.S. Pat. No. 3,682,730,particularly in columns 5 and 6, which patent is incorporated herein byreference.

The laminates of poly(p-methylstyrene) blend film with foamedpolystyrene or poly(p-methylstyrene) sheet are prepared as described inthe aforementioned U.S. Pat. No. 3,682,730, incorporated herein byreference. Either foamed polystyrene sheet or foamedpoly(p-methylstyrene) sheet can be used, so long as the film laminatedthereto is poly(p-methylstyrene) blend.

The processes of thermoforming to produce shaped articles are well knownto those skilled in the art. A generally preferred method ofthermoforming is vacuum forming. The shaped articles produced bythermoforming poly(p-methylstyrene) blend sheet can vary widely. Typicalshapes that are utilizable include trays, tubs, bowls, cups, and thelike. Such shaped articles are adaptable for packaging foods and, whenmade from poly(p-methylstyrene) blend and irradiated in accordance withthis invention, are adaptable for packaging foods containing fats.

Ionizing radiation is inclusive of extremely short-wavelength, highenergetic, penetrating rays such as gamma rays, X-rays, and subatomicparticles accelerated in cyclotrons, betatrons, synchrotrons, and linearaccelerators. The effect of irradiating the shaped articles is tocross-link the poly(p-methylstyrene) blend. The irradiation dose can bebetween about 5 megarads and about 30 megarads; whereaspoly(p-methylstyrene) requires between about 50 megarads and about 60megarads. Similar blends using polystyrene are not cross-linked at lowdosages.

EXAMPLES 1 THROUGH 6

A series of blends of poly(p-methylstyrene) (PPMS) with varying amountsof triallyl cyanurate or triallyl isocyanurate was prepared by meltblending in a Brabender mixer at 200° C. for 5 minutes. Each blend wasdissolved in methylene chloride (25 weight percent solution) and castwith a Gardner film applicator onto a polyethylene terephthalate sheet.The solvent was evaporated at 60° C. in a vacuum oven to yield 3-4 milfilms. The films were irradiated with an electron beam at dosages of 5,10 and 30 megarads. The weight percent of cross-linking was determinedby measuring solubility of irradiated films in toluene at roomtemperature. Insoluble gel found was filtered and dried to constantweight at 150° C. Pertinent data and results are set forth in the Table.

                  TABLE                                                           ______________________________________                                        Ex-                                                                           am-                   5        10     30                                      ple  Blend: PPMS +    megarads megarads                                                                             megarads                                ______________________________________                                        1    1% Triallyl cyanurate                                                                          11.1     13.4   68.1                                    2    2% Triallyl cyanurate                                                                          5.3      53.4   71.8                                    3    1% Triallyl isocyanurate                                                                       7.2      8.7    69.5                                    4    2% Triallyl isocyanurate                                                                       2.5      65.3   80.0                                    5    5% Triallyl isocyanurate                                                                       15.3     20.0   100.0                                    6.  10% Triallyl isocyanurate                                                                      16.8     63.0   99.9                                    ______________________________________                                    

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be resorted to, without departing from the spirit and scope of theinvention, as those skilled in the art will readily understand. Suchmodifications and variations are considered to be within the purview andscope of the appended claims.

What is claimed is:
 1. A shaped article of a blend ofpoly(p-methylstyrene) and between about 0.1 percent and about 10 percentof a polyunsaturated fatty acid sensitizer thermoformed and irradiatedwith ionizing radiation between about 5 megarads and about 30 megaradsto effect cross-linking to render the shaped article resistant to fattyfoods under microwave oven conditions.
 2. A shaped article of claim 1,wherein said blend contains between about 0.5 percent and about 5percent sensitizer.
 3. A shaped article of claim 2, wherein saidsensitizer is linseed oil.
 4. A method of making a shaped article thatis resistant to fatty foods under microwave oven conditions, thatcomprises forming a blend of poly(p-methylstyrene) with between about0.1 weight percent and about 10 weight percent of a polyunsaturatedfatty acid sensitizer into a foamed or unfoamed sheet, thermoformingsaid sheet to produce the shaped article, and irradiating the shapedarticle with ionizing radiation between about 5 megarads and about 30megarads to effect cross-linking to render the shaped article resistantto fatty foods under microwave oven conditions.
 5. The method of claim4, wherein said blend contains between about 0.5 percent and about 5percent sensitizer.
 6. The method of of claim 5, wherein said sensitizeris linseed oil.